The present invention relates to the lubrication of thermoplastic materials such as polyvinyl chloride (PVC), polypropylene (PP) and acrylonitrile-butadiene-styrene (ABS).
Lubricants are incorporated into plastics materials primarily to improve processibility of the material compounded by lowering melt viscosity and/or by preventing the polymer from sticking to the metal surface of the processing equipment. Lubricants may be classified functionally as either internal or external; or they may exhibit certain qualities attributable to both, and in such instances are classified as internal/external.
Internal lubricants are soluble in the polymer system and alter the cohesive forces between polymer chains so as to improve the flow of polymer chains past one another, thus lowering melt viscosity and improving flow properties.
External lubricants migrate to the surface of the polymer and provide lubrication between the molten and fused plastic composition and metallic parts of the processing equipment.
A good discussion of the function of plastic lubricants is found in A. P. Wilson's U.S. Pat. No. 3,981,838:
"In processing high melt viscosity polymers such as polyvinyl chloride (PVC) by extrusion, milling, calendering and injection molding, the shear forces applied cause excessive frictional heat which may lead to serious thermal stability problems. Another problem in processing PVC is to assure that the polymer releases from metal surfaces of the processing equipment. To solve these problems two types of lubricants are used. Lubricants which lower the melt viscosity and control frictional heat build-up are called internal lubricants while substances which promote release are called external lubricants. These materials are used in relatively small amounts since an excess will cause processing and stability problems and structural weakness in the ultimate product. In the processing of polymers such as PVC discrete particles are subjected to stress and heat until there is fusion of the discrete particles and a resulting loss of particle identity. An excess amount of an external lubricant will tend to coat the individual particles and while promoting a slippage between particles will delay fusion. PA1 The role of the external lubricant is to reduce the surface tackiness of the polymer, particularly during fabrication, thus reducing the tendency of the polymer to adhere to metallic surfaces. The desired degree of lubricity is something less than total release . . . . External lubrication . . . can be most easily established by measuring the time required for a lubricated polymer to exhibit sticking to the metal walls of a dynamic mill at elevated temperatures . . . . PA1 The role of the internal lubricant is to reduce the internal friction within the polymeric melt, which includes reducing heat build-up when the polymer is subjected to stress. Because of the characteristic high melt viscosities of rigid PVC an internal lubricant is usually viewed as being necessary to improve flow properties. Their use will result in an economic advantage in that less work will be expended at a given set of processing conditions. In addition, improved product appearance usually results, particularly with respect to improved surface appearance. An internal lubricant will promote fusion." PA1 Thermal degradation of the thermoplastic material within the extruder requiring a halt of process operations for clean out. PA1 Recycling of materials is limited. PA1 The use of thermoplastic materials in light colored goods is limited. PA1 High levels of expensive heat stabilizers may be required. PA1 EH: 2-ethylhexanoic acid PA1 C: commercial mixture of caprylic acid and capric acid PA1 S: commercial stearic acid PA1 O: commercial oleic acid PA1 B: commercial behenic acid.
Other distinguishing characteristics of internal and external lubricants are the effects they have on fusion time. Internal lubricants show no change in fusion time as the concentration of lubricant increases in the polymer system; external lubricants lengthen fusion time with increasing concentration.
Some lubricants exhibit properties of both internal and external lubricants and are identified as internal/external. The degree of each type of lubricity imparted in a specific application is dependent on the type and concentration of lubricant employed, the composition of the plastic system, the type of processing equipment, and the operating parameters of the processing system.
In some instances, one encounters undesirable side effects in the use of lubricants, most notably in the reduction of heat stability which can lead to such major production problems as:
Accordingly, it is the primary object of the present invention to provide lubricants for thermoplastic materials which possess good processing qualities without adversely affecting the heat stability of the processed material.
A further object of our invention is to provide a means for efficiently lubricating thermoplastic materials while minimizing degradation attendant to the heat of processing.
These and other objects will become more apparent from the discussion which follows.